The present invention relates to an image forming apparatus that conducts image forming operations according to the electrostatic copying process, such as a copier, a facsimile, a printer, etc.
In an image forming apparatus employing the electro-photographic method, such as a copier, a printer, etc., the electrostatic latent image is formed by applying an exposure processing to the circumferential surface of the photoreceptor drum uniformly charged in advance, so as to develop a toner image thereon by activating the developing device, and after that, the developed toner image is transferred onto the transfer material such as a recording paper sheet, etc. The transfer material onto which the toner image is transferred passes through the fixing device, to apply heat and pressure onto the transfer material so as to fix the toner image thereon.
In the image forming apparatus as abovementioned, the developing method (two component developing method), in which the developer constituting by toner and carrier (hereinafter, referred to as two component developer) is used for the operation for developing the electrostatic latent image formed on the photoreceptor drum, has been frequently employed, since the two component developing method is superior in the developing efficiency.
According to the two component developing method, by agitating the toner and carrier included in the developer concerned in the agitating chamber associated with the developing device, electric charges, generated by the friction electric charging action, are given to the toner from the carrier, so as to make the toner adhere onto the outer surface of the carrier. The carrier attached with toner, namely, the developer, is conveyed to the developing region located opposite to the photoreceptor drum by the developer conveyance member, for instance, like a developing sleeve that includes a magnetic roll therein. Then, the toner included in the developer is separated from the carrier at the developing region, and adheres onto a portion of the latent image formed on the photoreceptor drum so as to fog in the toner image. The residual toner and carrier remaining on the developer conveyance member are conveyed back to the agitating chamber, so as to reuse them for a next developing operation.
Since the toner stored in the agitating chamber is consumed and reduced according as the abovementioned developing process is repeated, the developing device employing the two component developing method is so constituted that new toner, being substantially equivalent amount of consumed toner, is supplied into the agitating chamber.
On the other hand, the carriers are reused without reducing its amount, even if the developing operation is repeated many times. However, by repeatedly agitating the carriers in the repeated usages of them, a resin coat applied on the circumferential surface of each of the carriers is gradually peeled off, and as a result, the resistance value of the carrier is decreased (resistance reduction phenomenon). Since the carrier whose resistance value has been decreased (hereinafter, referred to as the resistance reduced carrier, for simplicity) is liable to change its charging polarity when an electric charge is injected under the developing electric field, and accordingly, liable to move towards the photoreceptor member, there has been induced such a malfunction that the carrier is attached onto the solid image portion (portion to which a large amount of toner is to be adhered), or the like. Accordingly, it becomes necessary to remove the resistance reduced carriers residing within the agitating chamber therefrom.
To solve the abovementioned problem, the Tokkouhei 2-21591 (Japanese Patent Publication) sets forth a developing device employing, so called, the trickle method, in which, in the process of replenishing the agitating chamber with new toner being substantially equivalent amount of the consumed toner, new carriers associating with toner are supplied into the agitating chamber bit by bit, and by ejecting the developer overflowing from the agitating chamber when the toner and new carriers are supplied, the resistance reduced carriers residing within the agitating chamber are naturally ejected therefrom, so as to suppress the increase of the ratio of resistance reduced carriers included in the developer currently stored in the agitating chamber.
Further, Tokkai 2008-165061 (Japanese Patent Application Laid-Open Publication) sets forth a technology for selectively ejecting the resistance reduced carriers included in the developer, by making the resistance reduced carriers, included in the developer conveyed to the developing region by the developer conveyance member, selectively shift onto the photoreceptor member.
According to the technology, so called, the trickle method set forth in Tokkouhei 2-21591, for suppressing the increase of the ratio of the resistance reduced carriers included in the developer, since the developer to be abandoned includes not only the resistance reduced carriers, but also the new carriers newly supplied within a short past time, the replenishing efficiency would be deteriorated. Further, since resistance reduced carriers still remain within the developer, it has been difficult to prevent the reproduced image from suppressing occurrence of defects caused by the resistance reduced carriers, for instance, the occurrence of such the defect that some carriers adhered onto the solid color portion of the image concerned.
According to the technology set forth in Tokkai 2008-165061, by setting the skin potential (defined as a difference between the charge potential and the developing bias voltage) at a value larger than that to be employed for the normal image forming operation, the carriers, included in the developing agent conveyed to the developing region, is made to move toward the photoreceptor member. However, owing to its moving mechanism, it has been difficult to make only the resistance reduced carriers selectively move toward the photoreceptor member, and as a result, not only the resistance reduced carriers, but also non-deteriorated carriers are made to move toward the photoreceptor member, and therefore, the replenishing efficiency cannot be improved. Still further, since the residual resistance reduced carriers are still remain in the developer, it has been difficult to prevent the reproduced image from suppressing occurrence of defects caused by the resistance reduced carriers, for instance, the occurrence of such the defect that some carriers adhered onto the solid color portion of the image concerned. Yet further, since it is necessary to set the skin potential at a value larger than that to be employed for the normal image forming operation, there has arisen another problem that it is impossible to remove the resistance reduced carriers, while conducting the normal image forming operation.